A standard self-tightening drill chuck has a chuck body centered on and rotatable about an axis and formed with a forwardly open threaded bore and a holder sleeve rotatable about the axis on the body but axially generally nondisplaceable relative thereto and formed with a plurality of forwardly open angled guides angularly equispaced about the axis. Respective jaws are shiftable radially along the guides between inner and outer positions. A pusher threaded into the bore bears axially forward on the jaws and is angularly coupled to the jaws and holder sleeve so that relative rotation or the chuck body and pusher moves the jaws between their inner and outer positions.
A significant factor for the quality of a drill chuck consists in the amount of the clamping force that can be applied by the chuck to the tool or workpiece it holds in order to allow the transmission of high torque during drilling and to ensure safe clamping of the drilling tool between the inner edges of the jaws for the whole service life of the drill chuck, so that even if the drilling tool is frequently changed the jaws, whose inner edges are especially exposed to damage and preferably have a tapered profile, may become blunt if the clamping edges are deformed to the side. This may be caused involuntarily by the user when the tool diameters is very small or even when no drilling tool is present and the drill chuck is completely closed, for example by means of the power unit of the drill.
This is a particular problem when the inner edges of the jaws are formed with teeth intended to engage the tool in line or even point contact, since such teeth can bear on each other, in particular in a situation when the chuck is spun closed with no tool in it, with sufficient force to damage each other. The problem is particularly critical in systems where each jaw has a hard metal, e.g. cermet or carbide, insert that forms the tool-engaging teeth on the inner chuck face.